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Old 06-04-2007, 02:06 PM
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Tekknikal Tekknikal is offline
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Join Date: Apr 2007
Location: St Croix USVI
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Default Car Tech: Brakes 101



Having all of the power in the world is useless if it can't be managed. For our second Car Technology article, we will be dissecting the world of performance braking systems, as it would concern a car enthusiast who may be considering upgrading his or her own brakes. In this article, we will start with an overview of what brakes do, before going into all relevant components part by part. We will then conclude with entire systems and future technologies.



Brakes work by converting kinetic energy (motion) to thermal energy (heat) via friction. There are two main types of braking systems used on cars today. The first are drum brakes, and the second- which are becoming increasingly common- are disc brakes.

Drum brakes work by having a round enclosed piece of metal (the drum) rotating with the axle around brake shoes which are mounted inside of it. When you push the brake pedal, hydraulic fluid forces pistons in the brake cylinder (inside the drum) against the drum wall. The forced contact of the brake shoes against the drum wall creates friction which slows the axle down, bringing the car to a stop. Drum brakes have been around longer than disc brakes and are easier (read: less expensive) to produce. In addition, they make an emergency brake easy to implement. One key drawback to these brakes is that they generate heat very quickly- and are difficult to cool. This becomes critically important in high performance driving: if these brakes get too hot, they will no longer be able to create friction, and will lockup and fail.


(drum brake assemby)


(disc brake design)

Disc brakes are very similar to drum brakes in operation: hydraulic force pushes a cylinder against a brake pad, which makes contact with a rotating surface (the rotor), creating friction and thereby slowing the car. The key difference is that the cylinder is located within the brake caliper, which wraps around an iron disc- the rotor- which is connected to the drive axle. Some braking systems feature disc brakes at the front of the car and drum brakes at rear. Other cars feature disc brakes on all four wheels. In these four-wheel disc brake systems, the rear hubs usually contain an emergency brake that works like a drum brake at the center of the hub. In this article, we will focus on disc brake systems as these are the basis for all performance braking systems and are key to understanding future braking systems. Having said that, the key concepts of discussion here will largely apply to drum braking systems as well.

If what you're looking for is maximum performance from your brakes, you want to do four things: First, you want to be able to apply enough braking force to lock the spinning axle, at the vehicle's top speed. Second, you want to be able to absorb as much heat as possible in your braking system. Third, and in line with #2, you want to dissipate heat as quickly as possible, this allows you to stay under your heat capacity, keeping your brakes effective. Fourth, you want to ensure solid brake feedback. This gives you the confidence you need to push your car as hard as it can safely be pushed.

Allow me to elaborate on each point.


(Brembo manufactured racing setup)

The first point is pretty self explanatory. You need to apply enough force to force on a brake pad to force it against a rotor that is spinning at hundreds of revolutions per second. If you need to stop suddenly, you need to force that pad against the rotor with enough force to provide the friction you need. Although that's a lot of force, it's easily provided by most stock braking systems. Here's how it works: when you push the brake pedal, you're applying a force that gets multiplied via the master cylinder to a much greater hydraulic force. That force is then applied via your brake fluid to the pistons inside of the caliper. The caliper converts the pressure from the fluid to the movement of the pistons against the rotor. With correctly sized components, the necessary force is easily generated. In fact, most factory cars are already well equipped in this regard. Although some enthusiasts upgrade their master cylinders to achieve higher brake force, this isn't usually needed for a top notch non-professional braking system, even on the track.

As mentioned earlier, brakes work by converting the energy of motion into a different form of energy- heat. In ordinary physics, the amount of energy in a system is constant: so in slowing down a car, the amount of energy lost in the car's momentum must get completely converted to heat. Without getting into numbers, you can be rest assured that's a lot of heat! (enough to make the metal glow!) The braking system must be able to contain this heat. If it cannot, the braking system will not work and you would not be able to stop. For the braking system's components, this means that the materials used must be able to tolerate very high heat levels. In agressive street driving, it's possible for many brake components to see temperatures in several hundred degrees, especially on the surface of the rotor. On the track, conditions are even worse. The temperatures generated effect more than just the pads and rotors however- they also effect the caliper components and brake fluid. While metals can easily withstand a lot of heat, the same cannot be said for fluids. Fluids must be specially designed to withstand a lot of heat without evaporating...which would disconnect the cylinders from your brake pedal...meaning you cannot stop!

Even with all components in place, you will not be able to avoid generating more heat as time passes. With every turn you slow for, you will be generating more heat. In fact every time you use the brake pedal, no matter how lightly, heat is being created. To keep this heat level under the capacity for the braking system you must disappate that heat. There are a number of ways this is done, from ventilating the rotors to material choices and even component sizing.

Finally, you want the braking system to feel good. This means the components must react to your input in a natural and predictable way. Generally, you will want the brake pedal to be firm- you want it to drop only slightly when pressed, and then remain very firm, providing solid feedback. If you want to stop quicker, you want it to take more force, not more pedal travel.

What do these requirements mean for each of the key braking components? Read on to find out.

Performance Overview & Moding Goals


(master cyl. of ERD Infiniti G35)

Since the master cylinder is usually adequate, I will largely leave that out of this discussion. In some cases, a larger master cylinder is used to increase braking force, but again this is a relatively advanced modification who's precise effects are beyond the scope of this article. Just know that the master cylinder sizing directly relates to the brake force that's being applied.

Calipers

One of the most visible braking components is the caliper. The caliper is the metal assembly that wraps around the rotor, which as described earler- houses the pistons and pads. There are two main designs for calipers: fixed and floating (aka sliding).


(sliding caliper of an older generation Corvette)

Floating calipers are able to move with respect to the rotor: they have a piston on only one side. As you push the brake pedal, the piston pushes against the rotor until pads on both sides of the caliper come in contact with the rotor, which is where the brakes start to grab. A major advantage to this system is simplicity and cost. One downside to this system is that if the braking components corrode or get dirty enough (due to say, leaving the vehicle alone for long enough), dirt or debris might cause the pad connected to the caliper to always be in contact with the rotor- even when the brakes are released. This causes unnecessary wear and heat which could potentially damage other braking components.


(fixed front caliper of ERD Infiniti G35)

Fixed calipers offer a more efficient braking setup with improved feel over floating calipers. They work by fixing the location of the caliper relative to the disc and using one or more pairs of pistons (on opposite sides of the rotor) to clamp down on the rotor and provide braking force.

Generally, in high performance applications, fixed calipers are used. If you're upgrading your caliper, the upgrade will likely be to a fixed caliper design. The difference is apparent on sight- you will see the bulk from having a caliper containing pistons on the outside of the rotor. Note that this modification requires you to be careful with wheel selection: you will need to be particularly careful that your wheels will clear your caliper. On the street however, this modification is not really required to have a good braking system. However, it would mark the difference between good and excellent. It is also a modification that many like for asthetics, which brings up the size issue.

Bigger is not always better. I said it. Let's start with weight: the more weight you have on your car, (particularly here, as unsprung weight) the slower your car will be. This also reduces the effectiveness of your suspension and increases your braking distances. Some setups with 6, 8, or more pistons do so to allow larger pads which help increase pad life, but offer little real performance gain. Often, the difference in pad life may not be that major. Although the aggressive look goes without question: just know that it isn't always necessary.

Having said that, not all calipers are made equal. The caliper manages the pressure from the braking fluid and applies this to the pistons. One caliper from one manufacturer may be assembled to a higher level- providing better fitment, reliability, braking feel and feedback in comparison with another caliper. On the subject of fitment- one must also understand that a balanced brake system can consist of stock sliding calipers at rear and upgraded fixed calipers at front. This is OK, and in many cases a cost effective upgrade path: on most cars, a majority of the braking work is done on the front rotors. This is why, in general, the rear calipers and rotors can be smaller than the front. Weight can be saved without incurring any penalty except a slightly less aggressive look.

Rotors

The next key braking component are the rotors. These are what your brake pads will bite on to slow you down.
There are two main types of brake rotors: one peice and two peice. One piece rotors are just that- one piece from the area that the pad contacts to the hub. Two peice rotors use iron for the pad-swept area and aluminum or another light material for the hub area. The two sections are connected via pins mounted in elongated holes. This design offers a number of advantages over a single piece setup.

As a single piece setup gets hot, and the metal expands, there is nowhere for the metal to go, so the rotor as a whole tends to warp into a bowl like shape. As it cools, it will try to warp back, but won't really be able to as it is all one piece. Over time, the rotor will become permanently warped.


(2pc design of a Brembo Rotor assembly)

Two piece rotors solve this problem because when they get hot the swept area is allowed to expand- the bolts slide through their elongated holes. As they cool, they can resume their original shape. The benefits do not end there. Because the entire rotor doesn't need to be made of iron, lighter materials are used on the hat, saving extensive unsprung weight.


(stock one peice, ventillated rotor from an Infiniti G35)

Another design characteristic of rotors are their ventilated structure: some rotors are solid, while others have the inside hollowed out to allow air to flow around pillars. The benefits of this include improved heat capacity and improved heat dissipation. Many leading brake companies have proprietary designs for ventilation, to help optimize the airflow inside of the rotor, which further helps dissipate heat. In general, all two piece rotors are ventillated- often with proprietary ventilation technologies. One piece rotors however, tend to come in more different forms and you will find some that are not ventillated.

Yet another characteristic of rotors is the surfacing: some are plain, while others are slotted or drilled.


(One piece slotted rotors as seen on ERD Infiniti G35, note center is coated for rust protection)

The idea behind having slotted rotors is to help keep the rotor clear from debris and at the same time continually ensure good pad "bite". A drilled rotor provides the same benefit, but is more prone to cracking (note that all rotors- even plain- can crack).


(Cross drilled rotor with minor cracking- note this degree of cracking is safe)

On the street, the difference isn't likely to matter too much, and many prefer the look of drilled rotors. In extreme conditions however, slotted are recommended.

Pads

Brake pads are another important component in the braking system. Brake pads are designed to actually create the friction that slows the rotor and ultimately the vehicle. As such, the composition of brake pads are important- they must provide effective braking over a wide range of speeds (reverse to hundreds of miles per hour) and temperatures (-100 to over a thousand degrees F) without falling apart- that is to say, staying efficient. Originally pads were largely asbestos, but due to safety issues were given up for organic pads. Organic pads were improved on with semi-metallic pads. Today however, we have ceramic pads. Unlike semi-metallic pads which sacrifice noise for pad life, or vice-versa, ceramic pads allow you to have your candy and eat it too: due to their synthetic nature, they are designed to absorb more heat while being quieter (noise made is beyond human hearing), wearing slower, wearing the rotor slower, disappating heat faster, all while putting off lighter brake dust that is friendlier on both braking components and the wheels. Considering that prices aren't that great- this is a no brainier for performance oriented modern braking systems.

Lines and Fluid

Brake lines serve a very functional role- they allow hydraulic force to be used so you can stop the entire car with your foot. But there's more to the story- they are subjected to very high levels of pressure from the braking fluid. Most stock braking systems are comprised of high-strength rubber. This is because the material must be flexible enough to move with the suspension/wheel, but also be able to contain the brake force. If the lines are too flexible, when you try to brake, the pressure will cause the line to expand instead of force the pad on the rotor. The brake pedal would fall to the floor during this process and it would be a scary experience. Fortunately the department of transportation regulates these components, ensuring that your car has a capable braking system.

As you would imagine- there are ways to improve on stock rubber brake lines. Although the rubber lines that come with cars are very solid, they can often be replaced by braided steel lines.


(braided steel brake line on ERD Infiniti G35)

These lines wrap a solid rubber core with braided steel that prevents tiny levels of expansion that even the best stock rubber setups allow. This yields a firmer brake pedal and better brake feel. This allows for more confident braking.

The fluid that you use in your braking system is also very important, and is probably the most overlooked component in braking systems. Because the ideal braking system is capable of absorbing large amounts of heat and disappating that heat rapidly, the fluid used must be able to withstand that heat without starting to boil (that's when instead of pushing the pad against the rotor, you start to compress air in the line, and the pedal falls to the floor)...

There are four key classes of brake fluid: DOT 3, DOT 4, DOT 5, and DOT 5.1 (which are really DOT 4 fluids that meet DOT 5 requirements). DOT 5 is not compatible with DOT 3 and 4 systems and should never be mixed with them. Those fluids are also not well suited to performance and so will not be discussed here. DOT 3 and 4 are used by most street vehicles. The key thing to look for in picking a fluid is the dry boiling point. This is the temperature where the fluid (without contaminants) will boil. The higher the better. The wet boiling point is the temperature where the fluid contains water (3.7% to be specific). Again here, the higher the better.

Here are some examples of some top fluids being produced today, as well as their boiling points and stateside MSRPs:



































































Brake Fluid



Dry Boiling Point



Wet Boiling Point



Suggested List Price


AP SUPER 600 590F 410F $18.00/16.9oz. .
CASTROL SRF 590F 518F $69.99/33.8oz.
NEO SUPER DOT 610 610F 421F $15.00/12oz.
MOTUL RACING 600 593F 420F $15.00/16.9oz.
MOTUL DOT 5.1 509F 365F $6.50/16.9oz.
ATE SUPER BLUE 536F 392F $11.99/33.8oz
VALVOLINE SYNPOWER 503F 343F $4.97/16.9oz.
ATE SL 500F 329F $7.95/16.9oz.
CASTROL LMA 450F 311F $3.50/16.9oz.
AP 551 528F 288F $12.50/16.9oz.

(source: stoptech)

Note that Castrol SRF has an abnormally high wet boiling point, and for that reason is widely regarded as being in a class of its own- although you will pay for that. Many enthusiasts go with MOTUL RBF 600 or Super ATE. While the Motul RBF 600 fluid provides outstanding heat capacity, ATE Super Blue can be purchased in blue and a clear-ish color. This allows the user to tell when new fluid enters the system during bleeding.

Systems

With the above information one can evaluate the various components of his or her own braking system... but a good braking system is more than a number of components strung together. Everything must work together to provide a stable effective solution. That is, the system components must work together and the system needs to be balanced.



An unbalanced braking system can be a safety liability. To prevent this it is important to use braking systems and components that are designed to work with what you have. For example, because the front brakes do much more work than the rear, this means that oftentimes companies will provide a front braking upgrade for your car while the rears remain stock. In many street applications this is sufficient. Some people prefer to upgrade the rears as well for looks or if they simply need more than even the stock rear system can offer. Regardless of what exactly one's needs may be, there are a number of major, respected companies in the braking industry that provide aftermarket upgrades. Some of these include: AP Racing, Alcon, Brembo, Endless, StopTech, and Wilwood. While some are more recognized in one region vs another, all are recognized as major manufacturers who provide well engineered, reliable braking solutions.

Composite systems


(Brembo manufactured carbon braking system as seen on Mercedes-McLaren SLR)

Composite braking systems represent the future for braking performance. There are two leading technologies here: carbon/carbon and carbon/composite. These braking technologies are similar, but produced in different ways. The end results are a braking system with tremendous heat capacity, exceptionally fast heat disappation, and extremely long rotor life.




(Porsche Carbon Composite Brakes)

Porsche owns Carbon/Ceramic technologies while Brembo owns Carbon/Carbon technologies. Carbon/Carbon rotors are built by bonding epoxy resin with carbon layers for over 72 hours in an oven at temperatures of over 3,000 degrees. Carbon/Ceramic based rotors are built with silicon bonding materials for about 6 hours at much lower temperatures. Although the first Porsche systems with this option (called Porsche Ceramic Composite Brakes PCCB) had reliability problems - at high temperatures, the rotors would flake and crack- subsequent generations of rotors have largely resolved these issues. Brembo brakes have not been reported to have experienced this issue, although they have sometimes been criticized for lack of feel. Regardless, their performance has never been questioned and they're regarded as some of the world's best brakes. Fitted on cars such as the Ferrari Enzo and Mercedes-McLaren SLR, these brakes are only available on cars costing well over $100,000 at the moment. Per corner, this braking system costs well over $5,000. Porsche's PCCB system is available on many Porsche cars including the Boxster and Cayman, bringing the option to cars in the $60,000 range.


(Stoptech Composite Systems)

Although some track junkies still prefer steel, many are starting to warm to PCCB within Porsche's ranks. Long brake life, zero brake dust, and excellent thermal management are key reasons for their feelings. With these technologies continuing to advance daily, it may not be too long before these types of braking systems become options on more cars on the streets.


Summary


To conclude, braking works by converting the car's energy of motion to heat. That heat is must be contained within the braking system. The ideal braking system provides outstanding performance stop after stop, without fade. As a result, the ideal braking system would be able to dissipate heat rapidly enough to be ready for the next braking application. Achieving this level of performance on the street is often possible via small modifications to the braking system- depending on the car. For a higher level of performance, big brake kits are often necessary. Picking out the best performing braking system usually consists of the minimum components that it takes to get the job done, without adding unnecessary weight or complexity.
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